Introduction
Recently could hear a lot of questions about speakers and subwoofers. The vast majority of responses can be found on the first three pages of any book written by professionals. Material is addressed primarily to beginners, lazy ;) and rural Homebrew, prepared on the basis of books I.A.Aldoschinoy, V.K.Ioffe partly Ephrussi, journal publications in Wireless Worrld, AM and (some) personal experience. NOT used information from the Internet and Fidonet. The material is in no way purport to lighting problems, and is an attempt to explain the basics of acoustics on the fingers.
Most often, the question goes something like this: "the speaker found that to do with it?" Or "tovarischch and say such subwoofers are>." Here we consider only one solution to this problem: For existing dynamics to make a box, with the optimal settings for HCH as possible. This option differs from the task factory designer-pull system to lower the frequency of the required quantity of TU
[Q] Hashel large speaker on the occasion unmarked. How to know whether you can make him a subwoofer?
[A] Hujno measure its T / S parameters. Ha Based on these data to decide on the form of HCH clearance.
[Q] What is the T / S parameters?
[A] The minimum set of parameters for the calculation of HCH design proposed by Till and Small.
Fs-resonant frequency of the speaker without registration
Q factor Qts-full dynamics
Vas-equivalent volume of the speaker.
[Q] How to measure the T / S parameters?
[A] To do this, you need to collect from the generator circuit, a voltmeter, a resistor, and the test speaker. The speaker is connected to the output of the generator with an output voltage of several volts across the resistor of about 1 kOhm.
1. Remove V (F) = AFC resistance dynamics in the resonance region. The speaker must during this measurement to be in the free space (the distance from the reflecting surfaces). Hahodim speaker impedance on the current fasting person (useful), write in the air resonance frequency Fs (this is the frequency at which the voltmeter maximal :), the voltmeter Uo at minimum frequency (well, for example 10 Hz) and Um at the resonance frequency Fs.
2. Hahodim frequencies F1 and F2, in which the curve V (F) intersects the level of V = SQRT (Vo * Vm).
3. Hahodim Qts = SQRT (F1 * F2) * SQRT (Uo / Um) / (F2-F1) is the total quality factor of the dynamics, we can say, the most important value.
4. To find Vas need to take a small indoor Boxes volume Vc, with a hole a little smaller than the diameter of the diffuser. Tightly to lean to the speaker hole and repeat the measurements. From these measurements, you will need a resonant frequency dynamics in the housing Fc.Hahodim Vas = Vc * ((Fc / Fs) ^ 2-1).
This procedure is written in the audio shop • 4 for 99 year. I had checked it out .. There are others, when measured mechanical properties of the head, weight, flexibility, etc.
[Q] I now have options speaker, what to do?
[A] Each speaker in the design sharpen a certain kind of acoustic design. To find out under what is, look at the quality factor.
Qts> 1,2 it heads for the open boxes, optimally 2.4
Qts <0.8-1.0 - head for closed boxes, optimally 0.7
Qts <0.6 - for bass reflex, optimum - 0.39
Qts <0.4 - for horns
Wise to sort head is not on good quality and value for Fs / Qts. Cite from memory, the reluctance of the formula to calculate.
Fs / Qts> 30 (?) Screen and open housing
Fs / Qts> 50 closed body
Fs / Qts> 85 Tubes
Fs / Qts> 105 Bandpassy (bandpass resonators)
Elasticity, meatiness, dryness and other similar characteristics of the sound emitted by Bass speakers, are largely determined by the transient response of the system formed by the speaker, woofer design and the environment. To this system was not to release the impulse response, its quality factor should be less than 0.7 for systems with one side of the radiation dynamics (closed and phase inverter) and 1.93 for two-way systems (design type screen and open box)
[Q] Where to read about the open design?
[A] Open boxes and screens is the simplest type of decoration. Pros: ease of calculation, no increase in the resonant frequency (the size of the screen depends only view of the frequency response), almost constant quality factor. Disadvantages: large size of the front panel. Sufficiently competent and simple calculations of this type of design can be found in VK Joffe M.V.Lizunkov. Household speakers, M., Radio and Telecom.1984. And in the old Radio certainly have primitive amateur calculations.
[Q] How to calculate the closed box?
[A] Making "closed box" is of two types, infinite baffle and suspension compression. Getting into a particular category depends on the ratio of suspension dynamics and flexibilities in the air box, denoted alpha (by the way, you can try on the first and second count and change by filling).For an infinite screen ratio is less than 3 flexibilities for suspension compression more than 3-4. Can be considered as a first approximation that the heads with high Q sharpened by an infinite baffle, with less-under suspension compression. Taken to advance the speaker enclosure is closed infinite baffle has a larger volume than the compression box. (Generally speaking, when there is a speaker, the optimal body under it is clearly a certain amount. Errors encountered when measuring the parameters and calculations, can be slightly corrected by filling).Speakers for closed shells have powerful magnets and soft suspensions unlike heads for the open boxes. The formula for the resonant frequency of the dynamics in the design of volume V Fc = Fs * SQRT (1 + Vas / V), and an approximate formula relating the resonant frequency and quality of the head in the body (the subscript "c") and open space (subscript "s") Fc / Qtc = Fs / Qts
In other words, it is possible to implement the required quality factor only way speaker system, namely the choice of the volume of a closed box. What is the quality factor to choose? People who have not heard the sound of natural musical instruments, usually choose speakers with Q bolee1 0. In columns with a quality factor (= 1.0) the smallest uneven frequency response in the lower frequencies (and what does that sound?) Achieved at the cost of a small release on the transient response. Maximally smooth frequency response is obtained when Q = 0.7, and fully aperiodicity impulse response with Q = 0.5. Homogrammy for calculations can be taken in the above book.
[Q] Articles about speakers often words like "approximation of Chebyshev, Butterworth," etc. How does this relate to the speakers?
[A] The speaker system is a high-pass filter. The filter can be described by the transfer function. Transfer characteristic is always possible to fit a known function. In the theory of filters use several types of power functions, named for mathematicians first licked a particular function.Function determined by the order (maximum exponent, ie H (s) = a * S ^ 2 / (b2 * S ^ 2 + b1 * S + b0) has the second order) and the set of coefficients a and b (from these coefficients You can then move on to the values of the real elements of the electric filter, or electro-mechanical parameters.) Next, when it will go on the approximation of the transfer characteristic polynomial Butterworth or Chebyshev or something else, it should be understood so that the combination of the properties and dynamics of the body (or containers and inductance in an electric filter) turned so that most accurately the frequency and phase characteristics can be adjusted to a particular polynomial. Most smooth frequency response is obtained if it can be approximated by a polynomial Butterworth. Chebyshev approximation is characterized by the wave-frequency response and a large stretch of the working area (according to GOST to -14 dB) in the region of lower frequencies.
[Q] What kind of approximation to select the phase inverter?
[A] So before the construction of a simple bass reflex need to know the volume of the box and reflex tuning frequency (pipes, holes, passive radiator). If as a criterion to select the most smooth response (and this is not the only possible criterion), we get the following label A) Qts <0,3-most smooth curve will kvazitretego order B) Qts = 0,4 - better described battervortovskimi curves in) Qts > 0.5 - will allow the waves on the AFC, Chebyshev. In case A) bass reflex tuned to 40-80% higher than the resonance frequency in case b), the frequency of resonance, in case B) below the resonance frequency. Also in these cases will be a variable volume enclosure .. In order to find the exact frequency tuning, we must take the original formula, rather cumbersome to bring them here. Therefore, I refer interested in AudioMagazin 1999, then there is an educational program can be sorted out, or book Aldoshina. And even in the article Ephrussi Radio for '69 amiss.
Conclusion
If after reading all this, you still have the desire to rivet something yourself, you can take online nifty program type WinspeakerZ and calculate it all myself, remembering that of G.. candy not do. Not to be carried away by a decrease of the cutoff frequency, in any case, do not try to compensate for the roll off the amplifier. AFC can be leveled and a little bit, but the sound is enriched harmonics and subharmonics weight.On the contrary, the best results, in terms of pleasantness to the ear can be achieved forcibly ruining inlet UM very lowest frequencies, iefrequencies below the cutoff frequency HCH column. Another remark concerning the phase inverter configuration error in the resonance frequency of the phase inverter 20% leads to a surge or decline in response at 3 dB.
Oh, I almost forgot to say about subwoofers that actually strip resonators. Q-factor of the speakers for them should be even lower. The simplest bandpass too incalculable, but this ends my courtesy.
Original article sourse cxem.net
